Conventionally, exothermic elements in which an exothermic composition is housed in an air permeable covering material are widely used to warm a human body. For example, Patent Literature 1 discloses a steam exothermic element in which a powdered exothermic composition is housed in a highly air permeable bag member. In order to emit steam at a safe temperature from the steam exothermic element, the steam exothermic element is configured such that a temperature controlling material is provided between the exothermic composition and the bag member so as to provide a space between the steam exothermic element and the skin of the wearer, whereby the temperature of the steam that comes into contact with the skin is adjusted to 40° C. or more and 45° C. or less.
Patent Literatures 2 to 4 propose thin exothermic elements. These patent literatures disclose exothermic elements having a structure in which a stack including an exothermic composition slurry on a substrate sheet is covered with a covering material. The patent literatures also disclose that a water absorbing layer containing a water-retaining material is provided on the exothermic composition. In particular, Patent Literature 4 proposes a flexible and stretchable exothermic element that provides a sense of fit to the body surface by applying an exothermic composition slurry onto a covering material that is made of a highly air permeable non-woven fabric.
Aside from these techniques, in order to prevent excessive heat generation that can be caused by a large amount of exothermic composition being filled into one exothermic element for some reason or by a large amount of exothermic composition being unevenly distributed and concentrated in a particular area when an exothermic element is manufactured, the inventors of the present invention have proposed an exothermic element in which a water-retaining layer composed of a water-retaining sheet is stacked onto an exothermic layer so as to control the amount of water in the water-retaining layer (see Patent Literature 5).